TDMA mobile ad-hoc network (MANET) with second order timing and tracking

ABSTRACT

A communication system includes a plurality of mobile nodes forming a mobile ad-hoc network (MANET) and having a network clock time. A plurality of wireless communications links connects the mobile nodes together. Each mobile node includes a communications device and controller for transmitting and routing data packets wirelessly to other mobile nodes via the wireless communications link using a Time Division Multiple Access (TDMA) data transmission. Each mobile node includes a clock circuit having a digital clock time. A clock circuit is operative for processing a second order internal clock compensation factor as a learned and accumulated value for establishing a virtual clock time to correct any clock timing errors of the physical clock time from the network clock time.

FIELD OF THE INVENTION

The present invention relates to a mobile ad-hoc network (MANET), andmore particularly, the present invention relates to the tracking andtiming of clocks in a Time Division Multiple Access (TDMA) MANET andrelated methods.

BACKGROUND OF THE INVENTION

Mobile ad-hoc networks (MANET's) are becoming increasingly popularbecause they operate as self-configuring networks of mobile routers orassociated hosts connected by wireless links to form an arbitrarytopology. The routers, such as wireless mobile units, can move randomlyand organize themselves arbitrarily as nodes in a network, similar to apacket radio network. The individual units require a minimumconfiguration and their quick deployment can make ad-hoc networkssuitable for emergency situations. For example, many MANET's aredesigned for military systems such as the JTRS (Joint Tactical RadioSystem) and other similar peer-to-peer or Independent Basic Service SetSystems (IBSS).

TDMA technology is becoming more popular for use in these mobile ad-hocnetworks. In a TDMA ad-hoc network, channel access scheduling is a coreplatform of the network structure. Some problems, however, areencountered with distributed channel scheduling used in a multi-hopbroadcast networks. As known to those skilled in the art, the optimumchannel scheduling problem is equivalent to the graph coloring problem,which is a well known NP-complete problem, cited in numerous sources.Many prior art systems assume that the network topology is known and isnot topology transparent.

There is a changing topology in a TDMA ad-hoc network. Before thenetwork is formed, the topology cannot be learned. Without knowing thenetwork topology, the nodes in the network should still find a way tocommunicate. Once the nodes learn about the transmit and receiveschedules among neighboring nodes, these neighboring nodes may havemoved away, disappeared, or new nodes may have moved in. The rate ofresolving the scheduling must be fast and bandwidth efficient such thatthe network can be stabilized.

Nodes operative in a TDMA MANET typically use a crystal as part of itsclock. Each node's clock should be synchronized, but typically there issome deviation such that each node (or radio) could have a differentclock timing. This can occur even when higher quality crystals are used.

As a result, there can be a high network timing dispersion that requiresa long guard time. As a result, the time spent for a node to separatefrom the group of other nodes is limited by how fast its clock driftsfrom the other clocks in the group. For example, physical radios couldhave a different clock drift rate, which may also change withtemperature. Currently, some first order time tracking occurs where anew timing reference is tracked by an average of time frames ofneighboring nodes. Smoothing brings the network timing dispersion down,but its dispersion process does not stop. The time span for a node toleave the group without synchronization problems, however, is stilllimited by the divergence of the clock drift. A long guard time is stillrequired.

It is possible for an internal clock of a node to adjust periodically toa GPS time such that any network timing dispersion is minimized. GPSmust be equipped at each node, however, and a GPS signal is not alwaysavailable. Some proposals have a time server distribute a time stamp asa standard network clock, and all nodes resynchronize their internalclock to the new time stamp. The network timing dispersion resultingfrom clock draft, however, continues and a long guard time is required.

SUMMARY OF THE INVENTION

A communication system includes a plurality of mobile nodes forming amobile ad-hoc network (MANET) and having a network clock time. Aplurality of wireless communications links connect the mobile nodestogether. Each mobile node includes a communications device andcontroller for transmitting and routing data packets wirelessly to othermobile nodes via the wireless communications link using a Time DivisionMultiple Access (TDMA) data transmission. Each mobile node includes aclock circuit having a digital clock time. A clock circuit is operativefor processing a second order internal clock compensation factor as alearned and accumulated value for establishing a virtual clock time tocorrect any clock timing errors of the physical clock time from thenetwork clock time.

The virtual clock time can be established over multiple iterations. Theclock circuit within a mobile node can be operative for comparing aphysical clock time to a clock mean value that has been established forall mobile nodes within the MANET and establishing the virtual clocktime. A mobile node can also be operative for obtaining an average valuefor the physical clock within mobile nodes of the MANET. A mobile nodecan be operative for determining a physical clock error for a clock bysubtracting a mean value based on the calculated average value andscaling the virtual clock time that has been established by a correctionvalue.

In yet another aspect, the internal clock compensation factor can bebound by a decaying factor. The mobile node can be operative forcorrecting timing offsets with a reference to a common hypotheticalclock and calculating standard deviation at each measurement period ofthe timing offset. A timing reference can be propagated throughout thenetwork for synchronizing a plurality of mobile nodes.

In yet another aspect, the MANET can be formed by a source mobile node,destination mobile node, and plurality of neighboring mobile nodes. Theaverage value for the physical virtual clocks can be obtained withinneighboring mobile nodes and other factors established.

A method aspect is also set forth.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects, features and advantages of the present invention willbecome apparent from the detailed description of the invention whichfollows, when considered in light of the accompanying drawings in which:

FIG. 1 is a block diagram of an example of a communication system thatcan be used in accordance with non-limiting examples of the presentinvention.

FIG. 2 is a graph showing network timing dispersion in which nodes withdifferent clock drift rates will cause an increasing time discrepancy ifnot corrected.

FIG. 3 is a graph showing first order timing tracking in which nodeswith different clock drift rates will cause an increasing timingdiscrepancy such that nodes can resynchronize with each other to acommon time frame, periodically.

FIG. 4 is graph showing first order timing tracking with a standarddeviation of twenty-five nodes.

FIG. 5 is a graph showing second order timing tracking with a standarddeviation of twenty-five nodes.

FIG. 6 is a graph showing four nodes and clocks and showing an InternalClock Compensation Factor (ICCF).

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Different embodiments will now be described more fully hereinafter withreference to the accompanying drawings, in which preferred embodimentsare shown. Many different forms can be set forth and describedembodiments should not be construed as limited to the embodiments setforth herein. Rather, these embodiments are provided so that thisdisclosure will be thorough and complete, and will fully convey thescope to those skilled in the art. Like numbers refer to like elementsthroughout.

In accordance with a non-limiting example of the present invention, acommunications system uses second order timing tracking to reduce theguard time and increase the time spent for a node to leave the groupwithout synchronization problems. The divergence of a different clockdrift rate is minimized and is distributed as an adaptive scheme.

The system uses a virtual internal clock in each node with an internalclock compensation factor. The clock drift rate becomes adjustablebecause of a virtual clock drift rate. By using a second order timingfor tracking, a node can learn the environment, i.e., a neighboringnode's clock, and train itself. A node establishes a virtual networkclock and such nodes “virtually” have an identical clock even thoughphysical clocks may deviate. Thus, a node may drift away from the groupand drift away from the timing they used to have with the network, butthe node can come back and readjust, i.e., “learn” about the networkclock. With second order timing, the node adjusts based on a learningalgorithm.

An example of a communications system that can be used and modified foruse with the present invention is now set forth with regard to FIG. 1,followed by a description of a TDMA MANET and the system in accordancewith a non-limiting example of the present invention.

An example of a radio that could be used with such system and method isa Falcon™ III radio manufactured and sold by Harris Corporation ofMelbourne, Fla. It should be understood that different radios can beused, including software defined radios that can be typicallyimplemented with relatively standard processor and hardware components.One particular class of software radio is the Joint Tactical Radio(JTR), which includes relatively standard radio and processing hardwarealong with any appropriate waveform software modules to implement thecommunication waveforms a radio will use. JTR radios also use operatingsystem software that conforms with the software communicationsarchitecture (SCA) specification (see www.jtrs.saalt.mil), which ishereby incorporated by reference in its entirety. The SCA is an openarchitecture framework that specifies how hardware and softwarecomponents are to interoperate so that different manufacturers anddevelopers can readily integrate the respective components into a singledevice.

The Joint Tactical Radio System (JTRS) Software Component Architecture(SCA) defines a set of interfaces and protocols, often based on theCommon Object Request Broker Architecture (CORBA), for implementing aSoftware Defined Radio (SDR). In part, JTRS and its SCA are used with afamily of software re-programmable radios. As such, the SCA is aspecific set of rules, methods, and design criteria for implementingsoftware re-programmable digital radios.

The JTRS SCA specification is published by the JTRS Joint Program Office(JPO). The JTRS SCA has been structured to provide for portability ofapplications software between different JTRS SCA implementations,leverage commercial standards to reduce development cost, reducedevelopment time of new waveforms through the ability to reuse designmodules, and build on evolving commercial frameworks and architectures.

The JTRS SCA is not a system specification, as it is intended to beimplementation independent, but a set of rules that constrain the designof systems to achieve desired JTRS objectives. The software framework ofthe JTRS SCA defines the Operating Environment (OE) and specifies theservices and interfaces that applications use from that environment. TheSCA OE comprises a Core Framework (CF), a CORBA middleware, and anOperating System (OS) based on the Portable Operating System Interface(POSIX) with associated board support packages. The JTRS SCA alsoprovides a building block structure (defined in the API Supplement) fordefining application programming interfaces (APIs) between applicationsoftware components.

The JTRS SCA Core Framework (CF) is an architectural concept definingthe essential, “core” set of open software Interfaces and Profiles thatprovide for the deployment, management, interconnection, andintercommunication of software application components in embedded,distributed-computing communication systems. Interfaces may be definedin the JTRS SCA Specification. However, developers may implement some ofthem, some may be implemented by non-core applications (i.e., waveforms,etc.), and some may be implemented by hardware device providers.

For purposes of description only, a brief description of an example of acommunications system that could incorporate the second order timingtracking in accordance in accordance with a non-limiting example, isdescribed relative to a non-limiting example shown in FIG. 1. Thishigh-level block diagram of a communications system 50 includes a basestation segment 52 and wireless message terminals that could be modifiedfor use with the present invention. The base station segment 52 includesa VHF radio 60 and HF radio 62 that communicate and transmit voice ordata over a wireless link to a VHF net 64 or HF net 66, each whichinclude a number of respective VHF radios 68 and HF radios 70, andpersonal computer workstations 72 connected to the radios 68, 70. Ad-hoccommunication networks 73 are interoperative with the various componentsas illustrated. The entire network can be ad-hoc and include source,destination and neighboring mobile nodes. Thus, it should be understoodthat the HF or VHF networks include HF and VHF net segments that areinfrastructureless and operative as the ad-hoc communications network.Although UHF radios and net segments are not illustrated, these could beincluded.

The HF radio can include a demodulator circuit 62 a and appropriateconvolutional encoder circuit 62 b, block interleaver 62 c, datarandomizer circuit 62 d, data and framing circuit 62 e, modulationcircuit 62 f, matched filter circuit 62 g, block or symbol equalizercircuit 62 h with an appropriate clamping device, deinterleaver anddecoder circuit 62 i modem 62 j, and power adaptation circuit 62 k asnon-limiting examples. A vocoder circuit 62 l can incorporate the decodeand encode functions and a conversion unit could be a combination of thevarious circuits as described or a separate circuit. A clock circuit 62m can establish the physical clock time and through second ordercalculations as described below, a virtual clock time. The network canhave an overall network clock time. These and other circuits operate toperform any functions necessary for the present invention, as well asother functions suggested by those skilled in the art. Other illustratedradios, including all VHF mobile radios and transmitting and receivingstations can have similar functional circuits.

The base station segment 52 includes a landline connection to a publicswitched telephone network (PSTN) 80, which connects to a PABX 82. Asatellite interface 84, such as a satellite ground station, connects tothe PABX 82, which connects to processors forming wireless gateways 86a, 86 b. These interconnect to the VHF radio 60 or HF radio 62,respectively. The processors are connected through a local area networkto the PABX 82 and e-mail clients 90. The radios include appropriatesignal generators and modulators.

An Ethernet/TCP-IP local area network could operate as a “radio” mailserver. E-mail messages could be sent over radio links and local airnetworks using STANAG-5066 as second-generation protocols/waveforms, thedisclosure which is hereby incorporated by reference in its entiretyand, of course, preferably with the third-generation interoperabilitystandard: STANAG-4538, the disclosure which is hereby incorporated byreference in its entirety. An interoperability standard FED-STD-1052,the disclosure which is hereby incorporated by reference in itsentirety, could be used with legacy wireless devices. Examples ofequipment that can be used in the present invention include differentwireless gateway and radios manufactured by Harris Corporation ofMelbourne, Fla. This equipment could include RF5800, 5022, 7210, 5710,5285 and PRC 117 and 138 series equipment and devices as non-limitingexamples.

These systems can be operable with RF-5710A high-frequency (HF) modemsand with the NATO standard known as STANAG 4539, the disclosure which ishereby incorporated by reference in its entirety, which provides fortransmission of long distance HF radio circuits at rates up to 9,600bps. In addition to modem technology, those systems can use wirelessemail products that use a suite of data-link protocols designed andperfected for stressed tactical channels, such as the STANAG 4538 orSTANAG 5066, the disclosures which are hereby incorporated by referencein their entirety. It is also possible to use a fixed, non-adaptive datarate as high as 19,200 bps with a radio set to ISB mode and an HF modemset to a fixed data rate. It is possible to use code combiningtechniques and ARQ.

There now follows a general description of MANET TDMA processes ascommonly used, followed by a description of the second order timingtracking.

As is well known, ad-hoc network routing and data delivery is adifficult and complex problem. There are many different routingprotocols and methods used to solve different aspects of the networkissues. A background of the technology is given followed by adescription of second order timing tracking and tracking in accordancewith a non-limiting example of the present invention.

A Mobile Ad-hoc Network (MANET) can be described as an autonomous systemof mobile nodes. The network is typically self-organizing without theassistance from any centralized administration. Because there are nofixed and centralized base stations to maintain routes, the routingcapability is typically distributed to the individual mobile nodes. Eachnode is usually capable of discovering routes to a destination, and eachnode may also act as an intermediate node, i.e., a repeater, forforwarding the data packets in a multiple hop connection. The networktopology may change with time as the nodes move, enter, or leave thenetwork. Therefore, dynamic routing capabilities and route maintenancemechanisms are usually incorporated into the nodes.

There have been many different ad-hoc network protocols, which areusually divided into two different approaches, i.e., 1) proactive, and2) reactive. Proactive protocols, such as OLSR, CGSR, DBF, and DSDV,periodically send and exchange routing messages in the entire network tocatch up with the latest changes in the topology. Reactive protocols,such as ABR, DSR, AODV, CHAMP, DYMO, and TORA, however, search for aroute on-demand. A route discovery or route request message is typicallyflooded into the network upon request. As the request message comes tothe destination node, a route reply message, carrying the whole pathfrom the source to the destination, is transmitted back to the sourcenode.

Some protocols combine the two approaches, but in any event, the goal ofthe ad-hoc routing protocol is to find the current path, defined as asequence of intermediate nodes, from the source node to a destinationnode. Due to the changing topology and channel conditions, however, theroutes may have changed over time. Therefore, a route entry in therouting table may not be updated when it is about to be used. The routesmust be maintained either on demand or on a regular basis.

Routes can be maintained in two different levels. A first level is moreconcerned with the maintenance of the routing table, which is refreshedeither on a regular basis or on-demand. A second level is themaintenance of an actively used route path, which may have becomeunstable and unusable due to the node movement, blocking by objects,terrain conditions, and other link impairments. The source node shouldbe notified of the path errors, and another candidate route chosen or anew route discovery issued.

For table driven routing protocols, once a broken route is detected, itmay take some time for the protocol to react and resolve and find a newroute. Most link state based ad-hoc network protocols require aconvergence of routes in the route table. For example, in Optimized LinkState Routing (OLSR) protocol, a local route change would have to bebroadcast to all other nodes in the network such that in the routetable, the topology view is consistent. IT the route table is notconsistent, data packets may not be routed correctly. The data packetsare forwarded from hop to hop, originating from the source node towardsthe destination node. Due to the node movement, some of the intermediatenodes may have already moved out of the range of each other, thereforebreaking the path of delivery. Packets sometimes are dropped and thebroken path condition should be detected as soon as possible to formalternative paths.

For reactive ad-hoc network protocols, the route is typically discoveredon-demand. The nodes in the network keep track of the changes of thetopology, but only for the part on which they send traffic. Before datais sent, the destination path is discovered by sending a route request.It takes some time for the route request to travel to the destinationnode, which returns the path back to the source node. Explicit routepath information can be added to the packet header such thatintermediate nodes can forward the packet.

A data path can also be set-up in advance. A source node transmits apath label along a newly discovered route to the destination node. Theintermediate nodes remember the path label. Subsequently, the datapackets having a known label are forwarded correctly. Again, if the datapacket cannot be forwarded correctly along the path, the source node isnotified of the path error. The source node may issue a new routediscovery.

Some protocol provides local repair to a broken route. A repairing nodemay issue a locally bounded, limited path search downstream of the path.Due to the scope of a limited search, the response time is expected tobe faster. If it is successful, then the packets may flow through thedetour route. The repairing node would send a notification to the sourcenode about the change to the path. Local repair shortens this reactiontime to fixing the path failure. The mechanism, however, is notinstantaneous.

A data packet can typically be forwarded from the source node to thedestination node by two major methods. The forwarding decision can bemade by the source node such that explicit route information is attachedto the packet header. In the second method, the forwarding decision ismade by intermediate nodes. If the node has a view of the networktopology, the packet may be forwarded based on the routing table. If thenode has a label path established for labeled packets, the forwardingdecision can be based on the label of the packet. If the node has noknowledge of the network, and no established data path, the packet canbe flooded to all neighbors.

In a unidirectional link, however, the sender node may not know if thereceiver node actually received the packet. The sender node may have anexcessive number of packets in its transmission queue. The packet to bedelivered may be removed as if the packet is expired. The receiver nodeand the sender node may have moved apart further than the transmissionrange so that the packet can never by delivered via this specific link.The packets could be corrupted by signal fading or interference. ARQ(Automatic Repeat request) may be used to ensure a transmission successand a detection of a broken link. A significant delay may be incurred,however, waiting for an ACK and retransmission. Fault tolerance can beprovided using multiple paths to deliver the same set of packets. Moredata packets can be delivered with less delay, but some trade-off is theradio resource utilization that is significantly reduced.

In ad-hoc networks, nodes are equipped with limited radio resources anddata bandwidth. Data packets are typically classified according to theapplication requirement. Some applications require the data to bedelivered in a time critical manner, while other applications requirethe data to be delivered in a robust manner. It is important to deliverdifferent kinds of data packets differently and effectively according tothe demands imposed by the system. For example, dropping a few voicesamples is not as important as dropping a file data packet. Usually, afile data packet is less time critical, but it must be reliablydelivered.

Due to the issues of data delivery and Quality of Service (QOS)requirements, packets may be duplicated in multiple communication pathsso that the same packet has a higher chance of reaching its destinationin time. In many multi-path routing protocols, the source node maintainsa set of multiple communication paths as alternate routes in its routetable. It should be understood that multi-path routes can be discoveredin a similar fashion as a general route discovery.

Most of the generic route discovery mechanisms result in multiple pathswithout extra efforts. It is up to the source node to decide how manymulti-path candidates should be maintained in the route table. When thesource node is about to transmit a data packet to a destination withmulti-path routes, the node may duplicate data packets, each on aseparate member route of the multi-path, or the source node may use analternative path as a backup path in case the main path is notified asbroken. A higher level of fault tolerance can be achieved by sendingduplicated data packets. The multiple paths can be fully disjointed orpartially disjointed. A better fault tolerance can be served by thefully disjointed multiple paths. As multiple paths are used for faulttolerance, data packets are being forwarded redundantly on each memberroute of the multi-path. The network wide bandwidth consumption will beproportionally increased.

As set forth, there are a number of common terms used in the field. Forexample, a slot can be a basic TDMA time division structured by framesand slots. In each second, there are typically N number of frames, andin a frame, there are M number of time slots. Usually, each activemobile unit would have a chance, i.e., a time slot, to transmit in everyframe.

A frame can be considered as a general TDMA time division unit asexplained in reference to a slot.

A beacon can be a TDMA burst that is usually short and completed to oneslot. It could contain control information or controlled messages. In aHP-Net, a beacon can be transmitted in a slot. In a general TDMA scheme,it is typically transmitted in a generic time slot.

A beacon slot is typically the same as a slot.

In a 1-hop neighborhood, any neighboring node that is directly connectedwith a single link could be considered as the 1-hop neighborhood.

In a 2-hop neighborhood, any neighboring nodes that are directlyconnected with the maximum of 2-hops, 2 links away, could be consideredas a 2-hop neighborhood.

Network density could be referred to as the number of nodes in a per1-hop neighborhood, the number of nodes in a per 2-hop neighborhood, orthe number of nodes in a per geographical area.

A node could represent a mobile unit in a network topology.

Users are typically considered nodes and sometimes are also calledmobile users.

In channel access in TDMA, a channel can be defined by an exclusive useof a time slot in multiple frames. A node could be considered to have achannel when it is allowed to use a fixed time slot in all subsequentframes.

A channel collision could be a continuous slot collision in multipleframes. It usually results from more than one node trying to transmit inthe same net channel, which is the same slot in the frame.

Referring now to FIGS. 2-5, there now follows a description for thesecond order timing tracking system and method used, for example, inTDMA MANETS.

It should be understood that there is a network timing dispersion suchthat a long guard time is required. A time span for a node separate froma group is limited by how fast its clock is drifting away from thegroup's clock. Physical radios could have a different clock drift rate,which may also change with temperature. For example, as shown in FIG. 2,nodes A-D are represented by numerals 100, 102, 104 and 106respectively. Each node has its own internal clock. Each labeled blockrepresents one hour of each clock. In this example as illustrated, threehours later, the timing of the nodes is separated. Three o'clock p.m. isdifferent for each node, as illustrated.

Some proposals use smoothing as a type of first order timing andtracking to overcome such problems. A new timing reference is tracked byan average of time frames of neighboring nodes. Smoothing brings thenetwork timing dispersion down. The dispersion process, however, is notstopped. The time span for a node to leave the group withoutsynchronization problems is still limited by the divergence of the clockdrift, and a long guard time is still required.

In other solutions, the internal clock is adjusted periodically to a GPStime. Network time and dispersion as a result is minimized. A GPS devicemust be equipped at a node and a GPS signal is not always available.

FIG. 3 shows a fast clock at node A (100), a slow clock at node D (106)and resynchronization-smoothing 110. In this example, nodes withdifferent clock drift rates can cause an increase in timing discrepancy.The nodes can resynchronize with each other to a common (average,smoothing) time frame, periodically.

A network clock can be distributed. A time server can distribute a timestamp as the standard network clock. All nodes can resynchronize theirinternal clock to the new time stamp. The network timing dispersion dueto clock drift can still continue and a long guard time is required.

In accordance with a non-limiting example of the present invention, asecond order timing tracking is established that reduces the guard timeand significantly increases the time span for a node to leave the groupand come back without a synchronization problem. The divergence ofdifferent clock drift rates is minimized and is distributed with anadaptive scheme and is simple to implement.

The system and method in accordance with a non-limiting example of thepresent invention uses a virtual internal clock in each node. Thisestablishes an internal clock compensation factor (ICCF) that is added(processed) as a second order compensation factor to a resynchronizationalgorithm. The ICCF is a real multiplication factor and is a learned andaccumulated value over multiple iterations of correcting the timingerror. The virtual clock drift rate can equal the physical clock driftrate multiplied by the ICCF. Thus, the clock drift rate becomesadjustable.

FIGS. 4 and 5 are graphs showing timing and tracking of 25 nodes over aperiod of time. In these examples, the epoch timing of all nodes aresmoothed every second with the neighbors, such that each node has aboutfive neighboring nodes. The statistics of all 25 nodes' timing offset iscollected with reference to a common hypothetical clock. Standarddeviations can be calculated at each measurement period.

FIG. 4 shows a simple smoothing algorithm in which the timing differenceis tracked and kept to level of less than 100 ms.

FIG. 5 shows a second order tracking algorithm in which the timingdifference is diminishing over time because of the learned ICCF. Thevirtual clock at a node has a corrected clock drift from the physicalclock.

An example of second order timing tracking equations that can be used inaccordance with a non-limiting example of the present invention are nowset forth.

-   TR (A, k): Node-A, Timing reference at time kth second.-   TR (A, k, AVE): Node-A, Average Timing reference of all 1-hop nodes    at kth second.-   ICCF (A, k): Node-A, the Internal Clock Compensation Factor at kth    second.-   E (A, k): The timing “error” at kth second.-   ADJ_E (A, k): The timing adjustment at kth second.-   Alpha: Learning Roll off factor=0.8    E(A,k+1)=TR(A, k)−TR(A, k, AVE); error   1.    ADJ _(—) E(A, k+1)=E(A, k+1)*Alpha; adjusted error   2.    ICCF(A,k+1)={TR(A,k)−ADJ _(—) E(A, k+1)}/{TR(A, k)}  3.    Virtual Clock=Physical Clock*ICCF(A, k+1);result1   4.    TR(A, k+1)=TR(A,k)*ICCF(A, k+1);result 2   5.

FIG. 5 is another example of a timing graph showing four nodes 100(a) asnode-1; 102(b) as node-2; 104(c) as node-3; and 106(d) as node-4. Thetiming graph also shows a clock mean value (CMV) 120, a first orderadjustment (ADJ) 122, a physical clock period (PCP) 124 and virtualclock period (VCP) 126. Various equations and measurements areillustrated below the graph.

In that example shown in FIG. 5, the graph has four nodes 100-106 thatcould be generalized to many nodes. The horizontal bars indicate thephysical clock of each node and each has a different length thatrepresents the clock speed or clock growth rate. The vertical bar 130 isthe average value of all clocks, measured by a node, for example,node-4. In this example, node-4 could take its own physical clock periodand compare it to the clock mean value (CMV) and the difference is theerror (ERR). Node-4 could adjust its clock such that next time when themeasurement is made the error will be smaller. To be gradual and avoidisolations, the adjustment is scaled by a “correction pace” factor,alpha. Node-4 thus has a newly obtained virtual clock that is presentedas a “clock,” called VCP, which has a scaling factor related to its ownphysical clock period (PCP). The ICCF relates directly to the physicalclock. To prevent a group drift of ICCF in the network, it can befurther contained by a decaying factor, alpha 2. This decaying factorcould set the virtual clock as close as possible to the physical clocks.The last equation in FIG. 6 represents the ICCF decaying factor.

The system and method in accordance with a non-limiting example of thepresent invention reduces the guard time and reduces the effect of thedivergence of different clock drift rates in network synchronization. Itis a distributed and adaptive scheme and is simple to implement.Notably, it can reduce guard time in a TDMA system to increase bandwidthefficiency.

Many modifications and other embodiments of the invention will come tothe mind of one skilled in the art having the benefit of the teachingspresented in the foregoing descriptions and the associated drawings.Therefore, it is understood that the invention is not to be limited tothe specific embodiments disclosed, and that modifications andembodiments are intended to be included within the scope of the appendedclaims.

1. A communications system, comprising: a plurality of mobile nodesforming a mobile ad-hoc network (MANET) and having a network clock time;a plurality of wireless communications links connecting the mobile nodestogether; each mobile node comprising a communications device andcontroller for transmitting and routing data packets wirelessly to othermobile nodes via the wireless communications link using a time divisionmultiple access (TDMA) data transmission, each mobile node communicatingat a selected guard time and further comprising a clock circuit having aphysical clock time, wherein a clock circuit is operative for forestablishing a virtual clock time based on a timing error adjustmentobtained from a clock mean value subtracted from the physical clocktime, wherein said virtual clock time is processed with the physicalclock time to establish a second order internal clock compensationfactor as a learned and accumulated value and obtain a virtual clockdrift rate that is used to correct any clock timing errors of thephysical clock time from the network clock time and reduce the guardtime.
 2. The communications system according to claim 1, wherein saidvirtual clock time is established over multiple iterations.
 3. Thecommunications system according to claim 1, wherein a clock circuitwithin a mobile node is operative for comparing its physical clock timeto a clock mean value that has been established for mobile nodes withinthe MANET and establishing the virtual clock time.
 4. The communicationssystem according to claim 1, wherein a mobile node is operative forobtaining an average value for the physical clocks within mobile nodesof said MANET.
 5. The communications system according to claim 4,wherein a mobile node is operative for determining a physical clockerror for its clock by subtracting a mean value based on the calculatedaverage value and scaling the virtual clock time that has beenestablished by a correction value.
 6. The communications systemaccording to claim 5, wherein the internal clock compensation factor isbound by a decaying factor.
 7. The communications system according toclaim 5, wherein a plurality of mobile nodes within the MANET areoperative for collecting timing offsets with reference to a commonhypothetical clock and calculating standard deviations at eachmeasurement period of the timing offset.
 8. The communications systemaccording to claim 1, and further comprising a timing reference that ispropagated throughout the network for synchronizing a plurality ofmobile nodes.
 9. A mobile ad-hoc network (MANET), comprising: a sourcemobile node, a destination mobile node, and a plurality of neighboringmobile nodes, each mobile node comprising a communications device andcontroller for transmitting and routing data packets wirelessly to othermobile nodes using a time division multiple access (TDMA) datatransmission at a selected guard time; each mobile node furthercomprising a clock circuit having a physical clock time; and wherein aclock circuit is operative for establishing a virtual clock time basedon a timing error adjustment obtained from a clock mean value subtractedfrom the physical clock time, wherein said virtual clock time isprocessed with the physical clock time to establish a second orderinternal clock compensation factor as a learned and accumulated valueand obtain a virtual clock drift rate that is used to correct any clocktiming errors of the physical clock time from a network clock time andreduce the guard time.
 10. The communications system according to claim9, wherein said virtual clock time in a mobile node is established overmultiple iterations.
 11. The communications system according to claim 9,wherein a clock circuit within a mobile node is operative for comparingits physical clock time to a clock mean value that has been establishedfor neighboring mobile nodes and establishing the virtual clock time.12. The communications system according to claim 9, wherein a mobilenode is operative for obtaining an average value for the physical clockswithin neighboring mobile nodes.
 13. The communications system accordingto claim 12, wherein a mobile node is operative for determining aphysical clock error for its clock by subtracting a mean value based onthe calculated average value for physical clocks in neighboring mobilenodes and scaling the virtual clock time that has been established by acorrection value.
 14. The communications system according to claim 13,wherein the internal clock compensation factor is bound by a decayingfactor.
 15. The communications system according to claim 13, whereintiming offsets are collected by neighboring mobile nodes with referenceto a common hypothetical clock to calculate standard deviations at eachmeasurement period of the timing offset.
 16. The communications systemaccording to claim 9, and further comprising a timing reference that ispropagated throughout the network for synchronizing a plurality ofmobile nodes.
 17. A method for communicating between mobile nodes withina mobile ad-hoc network (MANET) using a time division multiple access(TDMA) data transmission, comprising: transmitting and routing datapackets wirelessly to other mobile nodes via wireless communicationslinks at a selected guard time, each mobile node comprising acommunications device, controller and a clock circuit having a physicalclock time; establishing a virtual clock time based on a timing erroradjustment obtained from a clock mean value subtracted from the physicalclock time; and processing the virtual clock time with the physicalclock time to establish a second order internal clock compensationfactor as a learned and accumulated value and obtain a virtual clockdrift rate that is used to correct any clock timing errors of thephysical clock time from a network clock time to reduce the guard time.18. The method according to claim 17, which further comprisesestablishing the virtual clock time over multiple iterations.
 19. Themethod according to claim 17, which further comprises comparing aphysical clock time within a mobile node to a clock mean value that hasbeen established for neighboring mobile nodes within the MANET andestablishing the virtual clock time.
 20. The method according to claim17, which further comprises obtaining an average value for the physicalclocks within mobile nodes of the MANET.
 21. The method according toclaim 20, which further comprises determining a physical clock error ina clock by subtracting a mean value based on the calculated averagevalue for physical clocks and scaling the virtual clock time that hasbeen established by a correction value.
 22. The method according toclaim 20, which further comprises collecting timing offsets withreference to a common hypothetical clock and calculating standarddeviations at each measurement period of the timing offset.
 23. Themethod according to claim 17, which further comprises propagating atiming reference throughout the network for synchronizing a plurality ofmobile nodes.